1. Field of the Invention
The present invention relates to a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, and a method for the transmission of said channel information.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
In the transmission of traffic by means of multi-carrier wave techniques, for example, orthogonal frequency division multiplexing (OFDM), the same number of bits are transmitted on all sub-carrier waves. This is done in spite of the fact that it is theoretically possible to transmit more bits if the channel is known, which is the case, for example, in the transmission of traffic on copper wire. It is, therefore, desirable to provide, in a multi-carrier transmission system, a method for the transmission of different numbers of bits per channel, or sub-carrier.
At the present time, bit-loading is used to vary the number of bits per channel, or sub-carrier, in so-called discrete multi-tone and OFDM-transmission both of which use multi-carrier wave techniques on known channels. Thus, such systems provide a method for the transmission of the correct number (real capacity) of bits per channel. However, while this method transmits a varying number of bits per channel, or sub-carrier, it would be advantageous to be able to transmit a fixed number of bits per channel, or sub-carrier, but with varying user data content.
It is an object of the present invention to provide a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represent a multiplicity of bits, and in which each symbol has a fixed maximum number of bits, the number of bits represented by a symbol, transmitted over a sub-carrier having a capacity less than said maximum value, being increased using channel coding bits.
It is another object of the present invention to provide, in a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represent a multiplicity of bits, a method for the transmission of said channel information.
According to a first aspect of the present invention, there is provided a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits, each of said transceivers including a receiver and a transmitter, characterised in that a fixed maximum value is determined for the number of bits for each symbol, and in that the system is adapted to determine the bit capacity per symbol of each of said plurality of sub-carriers, and to increase the number of bits represented by a symbol, transmitted over those sub-carriers having a capacity less that the fixed maximum value, to said maximum value by the addition of channel coding bits.
The fixed maximum value for the number of bits for each symbol may be determined on the basis of the bit capacity of that one of the sub-carriers having the highest theoretical bit capacity, and may be at least as large as the theoretical bit capacity of said one of the sub-carriers.
In accordance with the present invention, a multi-carrier transmission system is provided in which said transmission is effected by means of a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits, in which said system is adapted to determine a fixed maximum value for the number of bits for each symbol, said fixed maximum value being at least as large as the bit capacity of that one of said sub-carriers having the highest theoretical bit capacity, and in which said system is adapted to increase the number of bits represented by a symbol, transmitted over those sub-carriers having insufficient capacity, to said fixed maximum value by introducing a number of channel coding bits.
According to a second aspect of the present invention, there is provided, in a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits, each of said transceivers including a receiver and a transmitter, a method for the transmission of said channel information, characterised by determining a fixed maximum value for the number of bits for each symbol, determining the bit capacity per symbol of each of said plurality sub-carriers, and increasing the number of bits represented by a symbol, transmitted over those sub-carriers having a capacity less than the fixed maximum value, to said maximum value by the addition of channel coding bits.
In a preferred method, according to the invention, the fixed maximum value for the number of bits for each symbol is determined on the basis of the bit capacity of that one of the sub-carriers having the highest theoretical bit capacity. In accordance with this preferred method, the fixed maximum value for the number of bits for each symbol is at least as large as the theoretical bit capacity of said one of the sub-carriers.
The method of the present invention is characterised by the steps of transmitting the channel information by means of a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits, determining a fixed maximum value for the number of bits for each symbol, said fixed maximum value being at least as large as the bit capacity of that one of said sub-carriers having the highest theoretical bit capacity, and increasing the number of bits represented by a symbol, transmitted over those sub-carriers having insufficient capacity, to said fixed maximum value by introducing a number of channel coding bits.
The bit capacity of a sub-carrier may be determined by measurement, or estimation.
The channel coding bits may be used to decrease bit error rate and/or to validate said channel information at a receiver.
The multi-carrier transmission system may be a DMT system, or a DMT-based VDSL system.
According to a third aspect of the present invention, there is provided a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits, characterised in that said system uses a method as outlined in preceding paragraphs for the transmission of said channel information.